engine base. Sometimes when the separable head construction is employed the cylinder and part of the engine base are cast together, though this is not considered good practice except on small stationary power plants.

The Valve System.-The location of the valves and shape of the combustion chamber are factors that have a material bearing upon the power obtained from the gasoline motor. It is obvious that the gases must be admitted to and discharged from the cylinder as quickly as possible in order that no back pressure will be produced that will prevent the cylinder filling promptly or the burnt gases exhausting rapidly. Valve sizes also have a decided bearing upon the speed and power of the motor and some valve locations permit the use of larger members than do others without the penalty of using unsysmetrical cylinder castings. There are many ways in which valves may be placed in the cylinder and each system has some advantages. The cylinder shown at Fig. 38 is known as the "T" form and the valves are carried at opposite sides of the cylinders. It is necessary to provide two cam shafts for valve operation with this method of construction and theoretically this is the worst form of combustion chamber as far as heat efficiency is concerned. The cylinder of the "L" type, such as shown at Fig. 39 is very widely used because both valves may be actuated from a common cam shaft. The combustion chamber form is more efficient than the "T" head construction, though it is possible to use larger valves, and manifolds with the latter, than with the "L" construction.

The method of valve application shown at Fig. 42 has many advantages. It is possible to waterjacket the valves thoroughly, which is somewhat difficult to do when the valves are mounted in cages.

As the cooling

water is in contact with the walls of the valve chambers, there is no loss of heat conductivity, as there would be at the joint of the inserted cage type. The valves may be large and as there are no pockets it is possible to have a combustion chamber of the most efficient form. The cylinder is easily cast and machined and as the interior face of the cylinder head may be finished smooth the combustion chamber has no projecting particles which may serve as anchoring points for carbon or which may cause preignition troubles by getting red hot.

The method of valve operation and placing has much to do with the size of the valve, and the review of the various methods of valve installation has shown that the largest size valves can be used in motors of valve-in-thehead type. If valves are of the automatic type, i. e., open by the suction of the piston, they should be about 20 per cent. larger in diameter than mechanically operated valves. The valve diameter should be equal to half of the cylinder bore if possible. The valves with the larger area do not have to lift as high as the smaller members and they are quieter in action besides impeding the gas flow less.

There are two common methods of valve construction, one in which the valve head is made of cast iron and the stem of machinery steel attached to the head by riveting, the other where the valve is machined from a one-piece forging of nickel or machinery steel. Bevelseated valves are generally used in gasoline engines and most of these are provided with screw-driver slots on top of the head in order that they may be turned on their seat when grinding. Sometimes valves are made by electrically welding a nickel steel head to a carbon steel stem. This is considered a better method of construc

FIG. 44.-Front View of Power Plant With Timing Gear Case Cover Removed, Showing Conventional Method of Driving Cam Shaft.

tion than either of the two previously mentioned because there is no possibility of the valve heads becoming loose on the stem as is sometimes the case when a cast-iron head is riveted to a steel stem. It is preferable to the one-piece nickel steel construction because this material does not wear as well as does ordinary machinery steel. The factor of endurance is not slighted in order to use

the metal that has a high degree of resistance to heat. The composite valve construction has the advantage that the head is made of a heat-resisting material, while the stem is made of steel that will not wear the valvestem guide. As the head and stem are fused together by an electric current it is impossible to distinguish the joint and though made of different materials the valve is practically one piece.

Valves are operated by means of cams which may raise the valve from its seat through the medium of a simple plunger, as in the "T" and "L" head motors or by means of rocker arms and tappet rods used with valve-in-thehead motors. A cam is a small metal wheel having a raised projection at one point of the circle. This raises the valve plunger when it comes in contact with it. When the cam point is not in contact with the plunger there is a clearance between the plunger roller and the cam circle and the valve remains seated by virtue of spring pressure.' Cam shafts may be driven from the crank shaft by any form of positive gearing. Sometimes they are driven by means of silent chains and sprocket, though the usual method of drive is by gears. A typical timing gear assembly is shown at Fig. 44. In this the pinion A is driven by a gear of the same size attached to the crank shaft. It meshes with the gear B, which is twice as great in diameter, which turns the cam shaft at half engine speed. The small gear C is used to drive the water-circulating pump and mechanical oiler. Timing gears of the better class tractor engines are encased in order to exclude dirt and grit and retain lubricant.

Piston and Rings.-The piston is a cast-iron reciprocating member that fits into the cylinder and which receives the impact of the exploding gas. It is one of the simplest elements of the tractor power plant and is

one part that does not change much in form when used in different motor designs. It is a cylindrical member which has bosses in its interior to hold the wrist pin and grooves cut on the exterior wall in which cast iron packing rings are placed. Three forms of pistons are shown at Fig. 45. That at A is used in two-cycle motors and has a deflector plate or member cast integral with the head. That at B is the usual form used in the medium

FIG. 45.-Types of Pistons Commonly Used. A-Piston With Deflector for Two Cycle Engine. B-Piston With Four Rings and Connecting Rod in Place, Used on Medium Duty Engines. C-Long Piston of Heavy Duty Engine.

duty engines and is shown with connecting rod in place. That at C is a long piston, such as used in a heavy-duty engine. A piston with rings removed is shown at Fig. 46.

As it is imperative that the pistons be free in the